What are the possible genotypes of parents who produce a child with Turner Syndrome?

Turner syndrome is a chromosomal disorder that results in a chromasome complement of XO. The parents can have completely normal genotypes and still produce a child with Turner's syndrome. It is the gametes (either the sperm or the egg) of the parents that has an incorrect chromasomal complement which can be caused during meiosis of the gametes. I have a friend, who has Turner's syndrome, who's parents have totally normal genotypes.